An acid is a compound that when dissolved in water yields hydronium ions (H3O+) as the only positively charged ions.
A base is either a metallic oxide or a metallic hydroxide or an ammonium hydroxide which reacts with hydronium ions of an acid to form salt and water only.
A salt is a compound formed by the partial or total replacement of the ionizable hydrogen atoms of an acid by a metallic ion or an ammonium ion.